1. Field of the Invention
The present invention relates to a method for manufacturing a single-crystal silicon solar cell and a single-crystal silicon solar cell and, more particularly to a method for manufacturing a single-crystal silicon solar cell having a single-crystal silicon layer formed on a metal substrate and the single-crystal silicon solar cell.
2. Description of the Related Art
Solar cells produced using silicon as a principal raw material are classified into single crystal silicon solar cells, polycrystalline silicon solar cells, and amorphous silicon solar cells, depending on their crystallinity. Among these types, single crystal silicon solar cells are formed by slicing a single crystal ingot obtained by crystal pulling into wafers using a wire saw, processing each wafer to a thickness of 100 to 200 μm, and forming pn junctions, electrodes, a protective film, and the like on the wafer.
In case of the polycrystalline silicon, a polycrystalline ingot is manufactured by crystallizing molten metal silicon in a mold without pulling up a crystal, this is sliced into a wafer-like shape by a wire saw like the single-crystal silicon solar cell, and this is likewise processed into a wafer having a thickness of 100 to 200 μm, and p-n junctions, electrodes, a protective film, and others are formed, thereby obtaining a solar cell.
The amorphous silicon solar cell is obtained by decomposing a silane gas in a gas phase by discharge based on, e.g., a plasma CVD method to form an amorphous silicon hydride film on a substrate, adding diborane, phosphine, and others as a doping gas to be simultaneously deposited, and simultaneously effecting formation of p-n junctions and that of a film to form electrodes and a protective film. In the amorphous silicon solar cell, since amorphous silicon is of a direct transition type that absorbs an incident light, its optical absorption coefficient is approximately one digit higher (Kiyoshi TAKAHASHI, Yoshihiro HAMAKAWA, and Akio USHIROKAWA, “Taiyo-ko Hatsuden (Photovoltaic Power Generation)”, Morikita Shuppan, 1980, p. 233) than those of single crystal and polysilicon, and hence there is an advantage that approximately 1 μm that is approximately 1/100 of a film thickness of a crystal-based solar cell can suffice as a thickness of the amorphous silicon layer. In recent years, a production volume of solar batteries has exceeded one gigawatt per year in the world. Considering a further growth of the production volume in the future, an expectation about the thin-film amorphous silicon solar cell which can effectively exploit resources is considerable.
However, in manufacture of the amorphous silicon solar cell, a high-purity gas raw material such as silane or disilane is used as a raw material, or members other than a substrate are deposited in a plasma CVD apparatus. Under the circumstances, an effective utilization ratio of the gas raw material as a resource cannot be determined based on simple comparison with a film thickness required for a crystal-based solar cell. Further, a conversion efficiency of a crystal-based solar cell is approximately 15% whereas that of the amorphous silicon solar cell is approximately 10%, and hence a problem of degradation in output characteristic in application of a light still remains.
On the other hand, developing a thin-film solar cell by using a crystal-based silicon material has been attempted in many ways (Kiyoshi TAKAHASHI, Yoshihiro HAMAKAWA, and Akio USHIROKAWA, “Taiyo-ko Hatsuden (Photovoltaic Power Generation)”, Morikita Shuppan, 1980, p. 217). For example, a polycrystalline thin film is deposited on an alumina substrate or a graphite substrate by using, e.g., a trichlorosilane gas or a tetrachlorosilane gas. Since this deposited film has many crystal defects and a conversion efficiency remains low when this film is left as it is, zone melting must be carried out to improve crystallinity in order to raise the conversion efficiency (see, e.g., Japanese Patent Application Laid-open No. 2004-342909). However, even if such a method based on zone melting is adopted, there is a problem of, e.g., a reduction in photoelectric response characteristics in a long wavelength band due to a leak current at a crystal grain boundary or a decrease in a lifetime.